The technology of diode-pumped solid-state (DPSS) lasers has developed into a dominant driving force for laser technology worldwide in the past few years. Owing to the availability of high-power laser diodes, DPSS laser technology has the potential to replace a wide range of lasers commonly used in industrial and scientific applications as well as to create complete new fields for laser applications.
A growing commercial demand for ultraviolet lasers in recent years has resulted from the important physical fact that a shorter wavelength leads to a reduction of focal spot and image size. Therefore, lithographic structures can, for example, be reduced and information storage densities can be further increased. In the semiconductor industry, the ongoing change to 157 nm lithographic technology and smaller die size means that, to stay competitive, semiconductor manufacturers will have to purchase UV lasers with shorter wavelengths. This impacts primarily not only the sale of excimer lasers and UV optics, but also stimulates the development of new efficient and stable solid-state UV lasers worldwide to replace traditional gas lasers, which require very high power consumption and space but deliver beams of low quality.
The introduction of the first continuous-wave high-power ultra-violet laser at 266 nm in 1998 (see E. Zanger, R. Müller, B. Liu, M. Kötteritzsch, W. Gries, “Diode-pumped cw all solid-state laser at 266 nm”, OSA Trends in Optics and Photonics, Vol. 26, pp. 104-111 1999) and the continued engineering of this laser have re-newed or even triggered numerous applications in industrial fields. Good examples are DVD disc-mastering, wafer and mask inspection, fiber Bragg grating writing, circuit board inspection, confocal microscopy, 3D-prototyping, photolithography, capillary electrophoresis, micromachining, interferometric testing of optic production as well as holography, resonance Raman spectroscopy, laser trapping and cooling and isotope separation. Deep UV lasers of high power stability, low noise, excellent beam profile and long lifetime are the prerequisites for all industrial applications mentioned above. Furthermore, a compact laser with plug-in and hands-off operation is desirable in all applications above because all systems themselves may then be made more compact as well as plug & play. The diode-pumped all solid-state laser technology has made this possible.
Because of very limited wavelengths of useful laser crystals it is not possible to build up solid-state lasers in all desirable wavelength ranges. One of the most efficient ways is to use frequency conversion crystals to generate further desirable wavelengths, including the deep UV spectral range. Therefore, nonlinear process based frequency conversion crystals have become one of the key optical components in building up the desired laser systems.
Certain previous inventions are associated with concepts in designing key optical layouts e.g. doubling cavity, see DE 198 14 199, and in avoiding some physical effects having bad impact on the performance of the laser systems, e.g. photorefractive effect, see DE 198 15 362. All these inventions have laid the foundation for the first continuous-wave high-power ultraviolet laser at 266 nm.
It could have been also noticed that the degradation of some optics and crystals in the ultraviolet, especially in the deep ultraviolet regions deteriorates the performance and limit the lifetime of the designed laser systems. Such degradation phenomena are still quite popular in laser systems for the generation of high-power deep UV laser radiation.
Therefore, a laser capable of the elimination of degradation problems of optics and crystals would be desirable. Such a laser would have excellent output stability, low noise and long life-time, and would be simpler and cheaper to maintain in the industrial fields.